In the bending machines of the above-identified type the three rollers are set into rotation at the same angular speed, each about its own axis of rotation coinciding with its own geometric axis. Typically, the position of the axis of one of the three rollers (hereinafter referred to as “the stationary roller”) is fixed, whereas the axes of the other two rollers (hereinafter referred to as “the movable rollers”) can be moved with respect to each other and each with respect to the axis of the stationary roller to change the radius of curvature of the curved path imposed to the material to be bent. In this connection, there are various ways to change the relative position of the axes of the rollers.
For example, in the bending machine known from EP0919302 each of the two movable rollers is supported on a respective tilting arm which is actuated by a respective hydraulic actuator or cylinder to tilt about a stationary axis of rotation. Such a known solution does not allow to adjust the relative position of the axes of the rollers in a wide range, since the axis of each movable roller can be moved only along an arc of circumference the centre of which coincides with the tilting centre of the respective tilting arm.
WO2008/102388 also describes the possibility of displacing the axis of rotation of each of the two tilting arms along a vertical direction. To this end, each of the two tilting arms is hinged to the nut of a screw and nut linear actuation device, the screw of which has its own axis oriented vertically. If on the one hand this further known solution allows a wider range of adjustment of the relative position of the axes of the rollers, since it offers the degree of freedom of translation of the axis of rotation of each tilting arm in addition to the degree of freedom of rotation of the tilting arms, on the other hand it has a reduced stiffness, since the tilting arms are not hinged to structural parts of the machine, but to actuation devices which are clearly not able to provide a support having such a stiffness as that of a structural part of the machine. Such a known solution for adjusting the position of the axes of the rollers cannot therefore be used on large-size bending machines, which are required to apply high bending forces.